A friend of mine have got 3 goldfishes, a comet, a fantail and a black moor. She notice of late that her fishes are swimming on the top of portion of the tank. Mostly on the left hand side... is there a reason why? and a cause for concern?

I agree with Jaysee (often ). Goldfish produce large amounts of waste and can get very big. The fancy goldfish grow to around a foot, and the non-fancy varieties grow maybe twice as big. Comets are very fast swimmers and need a lot of horizontal swim space. I think the minimum tank those three fish should be kept in is a 70 gallon, and I think that most fishkeepers would say even bigger.

Your friend should strongly consider rehoming the fish. People have probably told her that a goldfish will only grow as big as its tank, and while this is (sort of) true, it is not good for the fish. Their growth is stunted, and this is very harmful.

No matter what, she's going to have trouble keeping the water parameters safe for the fish. They will produce so much waste that, IMO, even if she did very frequent water changes, she wouldn't be able to keep up. They are going to continue to be exposed to ammonia, eventually leading to death (a goldfish can live 25 years in appropriate conditions).

Another issue that could be causing problems is that goldfish are particularly susceptible to swim bladder issues and bloat, often caused by goldfish ingesting air when they try to eat floating foods. For this reason they should be fed sinking pellets rather than floating pellets or flakes. It's also not a good idea to keep fancy and non-fancy types of goldfish together, as the non-fancies are generally better swimmers and will out-compete the fancy goldfish for food.